There is no automated solution so every device has to be fixed manually. Plus with Bitlocker it becomes more difficult if the keys are stored on a server that can't start anymore. It'll be a pain and will probably last a month or two because of how big the scale is.
Depends, if you have a PXE boot setup, then you can use that to fix the machines. But I most people will be out of luck and do the manual route like you said